An important conventional process for oxidizing 2-mercaptobenzothiazole to benzothiazyl disulfide uses chlorine gas as the oxidizing agent, the chlorine being dispersed into a stirred aqueous solution of the sodium salt of the 2-mercaptobenzothiazole. The oxidation reaction using chlorine would normally be expected to follow the overall stoichiometry of the reaction as shown below: ##STR1##
The reaction equation shows that ideally no acidic or basic substance should be formed to change the pH of the aqueous mixture as chlorine is used to oxidize a solution of the sodium salt of 2-mercaptobenzothiazole to benzothiazyl disulfide. However, in actual practice and as taught in prior art, it is necessary to add alkali solution to the reaction mixture as the chlorine is added or to have present before hand excess alkali in a buffer in order to prevent the mixture from becoming acidic and consequently forming benzothiazyl disulfide contaminated with free 2-mercaptobenzothiazole. The amount of alkali required to hold the pH constant or prevent its dropping below about pH 8.5-9.5 is often as much as 0.3 to 1.0 molecular equivalent per mole of 2-mercaptobenzothiazole (as taught, for example, in U.S. Pat. No. 2,468,952 to Adolph J. Beber).
A study of the by-products formed, and the reaction rates leading thereto, during oxidation of 2-mercaptobenzothiazole with chlorine in aqueous alkaline mixture explains why additional alkali is necessary. Several water soluble "over-oxidized" products, primarily sodium benzothiazyl-2-sulfinate (I) and sodium benzothiazyl-2-sulfonate (II) are formed along with the disulfide during the oxidation. The "over-oxidation" reactions require additional chlorine usage and form hydrochloric acid. The "over-oxidation" reactions are shown below: ##STR2##
If no excess free alkali is added to the reaction mixture before or during chlorine addition, then the drop in pH occasioned by the formation of hydrochloric acid causes precipitation of free 2-mercaptobenzothiazole. The free thiol precipitates from its sodium salt at a pH below about 8.5 to 9.5. Contamination of the benzothiazyl disulfide product with the free 2-mercaptobenzothiazole is very undesirable in the use of the product as a vulcanization accelerator since the free thiol causes premature vulcanization at rubber compounding temperatures. The disulfide is very widely used as an accelerator because of its greater scorch protection, i.e. its lesser tendency to cause premature vulcanization or "scorch" in rubber vulcanizates. In practice a high purity product is desired, containing only a very small amount of the free mercaptan, generally less than 1 to 2 percent.
The requirement of high purity in the precipitated product disulfide has in the past been met by the addition of alkali to the reaction mixture to maintain a high pH, above 8.5-9.0 during substantially the entire chlorine addition. Adequate purity of product could be obtained, but the "over-oxidation" side reactions led to a yield loss of 7 to 10% of the starting 2-mercaptobenzothiazole as water soluble sulfinate and sulfonate salts. These side reactions use 40 to 60% of excess chlorine above stoichiometric requirements for the oxidation to benzothiazyl disulfide and require the aforementioned 0.3 to 1.0 molecular equivalents of alkali to neutralize the acidity formed by the over-oxidation reactions. It is possible to measure the efficiency of the oxidation process by analysis of the reaction slurry for dissolved benzothiazole sulfinates and sulfonates and chlorides, as well as dissolved, unoxidized 2-mercaptobenzothiazole.
In the aforementioned Beber process, under carefully optimized reaction conditions, such as vigorous agitation and fine dispersion of chlorine gas into the aqueous solution of sodium 2-mercaptobenzothiazole solution of about 5-6% concentration, it is necessary to use at least 0.2 moles of a base, e.g. sodium carbonate, to buffer the mixture adequately to prevent contamination of the benzothiazyl disulfide product with free 2-mercaptobenzothiazole. The final slurry contains precipitated benzothiazyl disulfide in about 92% yield; dissolved by-products total about 8%. The sodium chloride produced in the aqueous mixture is equivalent to 32 grams of chlorine consumed per 100 grams of product or 150% of the amount stoichiometrically expected from the simple reaction mechanism proposed hereinabove. If attempts are made to use lower levels of base, e.g. sodium carbonate, lower pH levels occur during the reaction and free mercaptobenzothiazole co-precipitates with and contaminates the product.
In a conventional batch precipitation process as taught in U.S. Pat. Nos. 2,265,344 and 2,468,952, the product quality, as represented by the level of contamination with free mercaptobenzothiazole in the benzothiazyl disulfide produced, can be strongly affected by the concentration of the starting solution of mercaptobenzothiazole. Use of mercaptobenzothiazole solutions greater than about 60 grams per liter leads to products with high mercaptobenzothiazole content (above 2%) even when the most vigorous agitation and thorough chlorine gas dispersion is employed. Use of additional excess alkali, with pH levels up to pH 10, and additional chlorine can lead to some reduction in the amount of contamination with the free mercaptobenzothiazole, but at a high cost in over-oxidation by-products, which are lost in the aqueous filtrate. Yields of less than 90% of benzothiazyl disulfide are then obtained. The necessity for low concentration of reactants requires that large volumes of aqueous solutions must be kept vigorously agitated during the entire time that chlorine is added to the batch, a period of about 2 hours under practical production operations. This requires very large corrosion resistant vessels made of glass, plastics, or high density wood, equipped with powerful agitators, to resist the severe corrosive effects of wet chlorine gas.
In the conventional batch chlorine oxidation process the results are markedly affected by the degree of purity of the 2-mercaptobenzothiazole solution used. In normal large scale commercial operations this solution is ordinarily derived from a relatively crude reaction product of aniline, carbon disulfide and sulfur, as described in U.S. Pat. No. 1,631,871 to Kelly or modifications thereof, by extraction with caustic soda. Various methods are employed to attempt to minimize the carryover of tarry by-product impurities which have some solubility in the sodium mercaptobenzothiazole solution. These residual impurities, often present as 2-3% of the mercaptobenzothiazole, tend to co-precipitate as soft tarry droplets along with the benzothiazyl disulfide and, by occluding the particles of benzothiazyl disulfide, form granular oversized particles known in the art as "sand." This "sand" must be separated from the slurry of benzothiazyl disulfide before further processing because it interferes with milling and screening of the product and causes non-uniform dispersion of benzothiazyl disulfide when compounded into rubber stock as a vulcanization accelerator. The appearance of these sand-like impurities has always been a vexing problem in the production of benzothiazyl disulfide by oxidations of aqueous alkaline solutions of 2-mercaptobenzothiazole and various means have been tried to minimize the contamination. In the Beber process the sodium mercaptobenzothiazole solutions are first purified by precipitation of impurities from the alkaline solution of commercial grade 2-mercaptobenzothiazole of apparent 94-95% purity. In U.S. Pat. Nos. 2,349,599 to Moorhouse, 2,730,528 to Weyker and 3,131,196 to Wood, methods, for purifying solutions of 2-mercaptobenzothiazole as derived from the Kelly process, are suggested; in U.S. Pat. No. 2,830,058 a method for inhibiting the "sand" impurity formation is suggested. In practice none of these methods have proven completely satisfactory. Extra purification steps and more materials are needed and there is a loss in yield of benzothiazyl disulfide based on the starting 2-mercaptobenzothiazole content of the original crude material.